bug#32189: 27.0.50; GCC 7 warning due to -Wformat-truncation=2

[Eli Zaretskii <eliz@gnu.org>]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Thu, 19 Jul 2018 08:10:20 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>, 32189@debbugs.gnu.org
> 
>  This doesn't work with GCC 7.  (Maybe it would work with GCC 8; the 
>  release notes say that it is better at avoiding false positives.)  For 
>  integer specifiers like "%ld", the only thing I've found that works 
>  without enlarging the buffer is to cast the argument to a smaller 
>  integer type. 
> 
> You can't do that; %ld requires a long argument, and casting results in undefined behavior.

We can assign instead, and since the value is known to be small
enough, doing that won't lose bits.